Activation of endothelial cell kinin receptors leads to intracellular calcium increases and filamin translocation: regulation by protein kinase C

G protein-coupled receptors directly bind filamin A with high affinity and promote filamin phosphorylation

Although interaction of a few G protein-coupled receptors (GPCRs) with Filamin A, a key actin cross-linking and biomechanical signal transducer protein, has been observed, a comprehensive structure-function analysis of this interaction is lacking. Through a systematic sequence-based analysis, we found that a conserved filamin binding motif is present in the cytoplasmic domains of >20% of the 824 GPCRs encoded in the human genome. Direct high-affinity interaction of filamin binding motif peptides of select GPCRs with the Ig domain of Filamin A was confirmed by nuclear magnetic resonance spectroscopy and isothermal titration calorimetric experiments. Engagement of the filamin binding motif with the Filamin A Ig domain induced the phosphorylation of filamin by protein kinase A in vitro. In transfected cells, agonist activation as well as constitutive activation of representative GPCRs dramatically elicited recruitment and phosphorylation of cellular Filamin A, a phenomenon long known to be crucial for regulating the structure and dynamics of the cytoskeleton. Our data suggest a molecular mechanism for direct GPCR-cytoskeleton coupling via filamin. Until now, GPCR signaling to the cytoskeleton was predominantly thought to be indirect, through canonical G protein-mediated signaling cascades involving GTPases, adenylyl cyclases, phospholipases, ion channels, and protein kinases. We propose that the GPCR-induced filamin phosphorylation pathway is a conserved, novel biochemical signaling paradigm.

Characterization of dual agonists for kinin B1 and B2 receptors and their biased activation of B2 receptors

Kinin B1 and B2 receptors (kB1R and kB2R) play important roles in many physiological and pathological processes. In some cases, kB1R or kB2R activation can have overlapping or complementary beneficial effects, thus an activator of both receptors might be advantageous. We found that replacement of the C-terminal Arg in the natural kB2R activators bradykinin (BK) or kallidin (KD) with Lys (K(9)-BK or K(10)-KD) resulted in agonists that effectively stimulate the downstream signaling of both the kB1R and kB2R as measured by increased inositol turnover, intracellular calcium, ERK1/2 phosphorylation, arachidonic acid release and NO production. However, K(9)-BK and K(10)-KD displayed some characteristics of biased agonism for kB2Rs as indicated by the rapid kinetics of ERK1/2 phosphorylation induced by K(9)-BK or K(10)-KD compared with the prolonged response mediated by BK or KD. In contrast, kinetics of ERK phosphorylation stimulated by K(10)-KD activation of the kB1R was the same as that induced by known kB1R agonist des-Arg(10)-KD. Furthermore, the endocytosis of kB2Rs mediated by K(9)-BK and K(10)-KD was remarkably less than that induced by BK and KD respectively. K(10)-KD stimulated kB1R and kB2R-dependent calcium responses and ERK1/2 phosphorylation in bovine endothelial cells. In cytokine-treated human endothelial cells, K(10)-KD stimulated ERK1/2 phosphorylation and a transient peak of NO production that was primarily kB2R-dependent. K(10)-KD also stimulated prolonged NO production that was both kB1R and kB2R-dependent. These data provide the first examples of dual agonists of kB1R and kB2R, and a biased agonist of kB2R and may provide useful clues for developing dual modulators of kB1Rs and kB2Rs for potential therapeutic use.

Melatonin inhibits endothelial nitric oxide production in vitro

Endothelial cell function is a major player on the regulation of both vascular tonus and permeability. Activation of nitric oxide synthase (NOS) by bradykinin is one physiological pathway for the well-known vascular relaxation mediated by endothelial-derived nitric oxide (NO). In this study we investigated if melatonin, which is known to modulate endothelial cell function and NO production in other tissues, is able to impair bradykinin-induced NO production in vitro. Rat microvascular endothelial cells were incubated with fluorescent dyes to detect either NO or Ca2+. In addition, cGMP levels were measured by enzyme immunoassay. We found that while bradykinin (1-100 nm) increased both cytosolic Ca2+ and NO production, melatonin (1 nm) abolished this NO production but not cytosolic Ca2+ elevation. N-acetylserotonin (0.1 and 1 nm) had the same effect, while the selective agonist for MT3 receptors (5-MCA-NAT, 1 nm) had no effect. Moreover, nonselective and MT2-selective antagonists did not alter the effect of melatonin, suggesting that it is not mediated by MT melatonin receptors. A possible direct inhibition of calmodulin was also discarded as melatonin did not mimic the effect of calmidazolium on cytosolic Ca2+. Melatonin also abolished cGMP production induced by 1 microm bradykinin, indicating that the NO downstream effect is impaired. Thus, here we show that melatonin reduces NO production induced by bradykinin by a mechanism upstream to the interaction of Ca2+ -calmodulin with NOS. Moreover, this effect might be the basis of the diurnal variation in endothelial cell function.

Regulation of endothelial cell barrier function by calcium/calmodulin-dependent protein kinase II

Thrombin-induced endothelial cell barrier dysfunction is tightly linked to Ca(2+)-dependent cytoskeletal protein reorganization. In this study, we found that thrombin increased Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase II) activities in a Ca(2+)- and time-dependent manner in bovine pulmonary endothelium with maximal activity at 5 min. Pretreatment with KN-93, a specific CaM kinase II inhibitor, attenuated both thrombin-induced increases in monolayer permeability to albumin and decreases in transendothelial electrical resistance (TER). We next explored potential thrombin-induced CaM kinase II cytoskeletal targets and found that thrombin causes translocation and significant phosphorylation of nonmuscle filamin (ABP-280), which was attenuated by KN-93, whereas thrombin-induced myosin light chain phosphorylation was unaffected. Furthermore, a cell-permeable N-myristoylated synthetic filamin peptide (containing the COOH-terminal CaM kinase II phosphorylation site) attenuated both thrombin-induced filamin phosphorylation and decreases in TER. Together, these studies indicate that CaM kinase II activation and filamin phosphorylation may participate in thrombin-induced cytoskeletal reorganization and endothelial barrier dysfunction.

Structural and functional aspects of filamins

Filamins are a family of high molecular mass cytoskeletal proteins that organize filamentous actin in networks and stress fibers. Over the past few years it has become clear that filamins anchor various transmembrane proteins to the actin cytoskeleton and provide a scaffold for a wide range of cytoplasmic signaling proteins. The recent cloning of three human filamins and studies on filamin orthologues from chicken and Drosophila revealed unexpected complexity of the filamin family, the biological implications of which have just started to be addressed. Expression of dysfunctional filamin-A leads to the genetic disorder of ventricular heterotopia and gives reason to expect that abnormalities in the other isogenes may also be connected with human disease. In this review aspects of filamin structure, its splice variants, binding partners and biological function will be discussed.

Postischemic anti-inflammatory effects of bradykinin preconditioning

We sought to determine the mechanisms whereby brief administration of bradykinin (bradykinin preconditioning, BK-PC) before prolonged ischemia followed by reperfusion (I/R) prevents postischemic microvascular dysfunction. Intravital videomicroscopic approaches were used to quantify I/R-induced leukocyte/endothelial cell adhesive interactions and microvascular barrier disruption in single postcapillary venules of the rat mesentery. I/R increased the number of rolling, adherent, and emigrated leukocytes and enhanced venular albumin leakage, effects that were prevented by BK-PC. The anti-inflammatory effects of BK-PC were largely prevented by concomitant administration of a B(2)-receptor antagonist but not by coincident B(1) receptor blockade, nitric oxide (NO) synthase inhibition, or cyclooxygenase blockade. However, NO synthase blockade during reperfusion after prolonged ischemia was effective in attenuating the anti-inflammatory effects of BK-PC. Pan protein kinase C (PKC) inhibition antagonized the beneficial effects of BK-PC but only when administered during prolonged ischemia. In contrast, specific inhibition of the conventional PKC isotypes failed to alter the effectiveness of BK-PC. These results indicate that bradykinin can be used to pharmacologically precondition single mesenteric postcapillary venules to resist I/R-induced leukocyte recruitment and microvascular barrier dysfunction by a mechanism that involves B(2) receptor-dependent activation of nonconventional PKC isotypes and subsequent formation of NO.

NFkappaB translocation in human microvessel endothelial cells using a four-compartment subcellular protein redistribution assay

Protein distribution profiles may be used to characterize both physiological and pathophysiological cellular changes, but rigorous biochemical assays for measuring such movements are lacking. This paper reports on a protein redistribution assay that combines reversible metal chelate-based total protein detection with a four-fraction subcellular detergent fractionation procedure. TNF-alpha stimulated cultured human omental microvessel endothelial cells are fractionated into cytosol, membrane/organelle, nuclear (envelope and associated), and cytoskeletal/DNA compartments. Protein fractions are separated electrophoretically and electroblotted or slot-blotted onto PVDF membranes without electrophoretic separation. A key feature is that total protein is measured and analyzed directly on the resultant PVDF membrane, using a Ferrozine/ferrous metal-chelate stain, without the added step of a prior solution-phase protein assay. As a result, factors that may adversely affect NFkappaB quantification, such as saturation of the solid-support membrane, are rigorously evaluated and controlled. Following removal of the Ferrozine/ferrous total protein stain, NFkappaB distribution is determined via standard immunodetection procedures. This assay reveals a new level of complexity regarding NFkappaB distribution and translocation. NFkappaB is shown to translocate from the cytosol to the membrane/organelle and cytoskeletal/DNA fractions, whereas trace levels of NFkappaB are observed in the nuclear (envelope and associated) fraction. Dose-curve analysis reveals that the response is initiated at 10 U/ml of TNF-alpha, plateaus at approximately 1000 U/ml, and remains essentially constant up to 2000 U/ml. Time-course analysis demonstrates a measurable response as early as 5 min and a peak response at approximately 30 min, after which the distribution begins to return to baseline. The assay should provide a valuable tool for rapid evaluation and mechanistic studies of NFkappaB redistribution.